CVE-2021-38387
📋 TL;DR
This vulnerability in Contiki 3.0's Telnet server causes connected clients to enter an infinite loop when the server silently quits before disconnecting. This leads to excessive CPU consumption on affected devices. Systems running Contiki 3.0 with Telnet server enabled are vulnerable.
💻 Affected Systems
- Contiki OS
📦 What is this software?
Contiki by Contiki Os
⚠️ Risk & Real-World Impact
Worst Case
Denial of service causing complete system unavailability due to CPU exhaustion, potentially affecting critical IoT/embedded systems.
Likely Case
Degraded performance and service disruption on affected devices, requiring manual intervention to restore functionality.
If Mitigated
Minimal impact if Telnet is disabled or proper monitoring detects abnormal CPU usage patterns.
🎯 Exploit Status
Exploitation requires network access to Telnet port (typically 23). No authentication needed to trigger the condition.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Contiki 3.x with fix from GitHub issue #2688
Vendor Advisory: https://github.com/contiki-os/contiki/issues/2688
Restart Required: Yes
Instructions:
1. Update Contiki OS to version with fix from GitHub issue #2688. 2. Recompile and redeploy affected firmware. 3. Restart devices with updated firmware.
🔧 Temporary Workarounds
Disable Telnet Server
allDisable the Telnet server functionality in Contiki configuration
Modify project-conf.h to disable TELNETD: #define TELNETD 0
Network Segmentation
linuxRestrict access to Telnet port (23) using firewall rules
iptables -A INPUT -p tcp --dport 23 -j DROP
🧯 If You Can't Patch
- Implement network monitoring for abnormal CPU usage patterns on Contiki devices
- Deploy rate limiting or connection limits to Telnet service
🔍 How to Verify
Check if Vulnerable:
Check if running Contiki 3.0 with Telnet server enabled. Test by connecting to Telnet port and monitoring client behavior when server terminates unexpectedly.Check Version:
Check Contiki version in source code or firmware metadataVerify Fix Applied:
After patching, test Telnet connections and verify clients disconnect properly when server terminates.📡 Detection & Monitoring
Log Indicators:
- High CPU usage logs from Contiki devices
- Telnet connection anomalies
- Process hanging indicators
Network Indicators:
- Multiple stalled Telnet connections
- Unusual traffic patterns to port 23
SIEM Query:
source="contiki" AND (cpu_usage>90 OR process="telnetd")